As dimensions for microelectronics are reduced and the demand for such materials increase, thin semiconductor films having uniform composition and uniform thickness over a larger substrate area are increasingly desirable. A common process used for depositing thin films on a substrate is Chemical Vapor Deposition (CVD), which provides for deposition of relatively uniform films over complex device topography. In a typical CVD process, the substrate is exposed to two or more volatile precursors, which react and/or decompose on the substrate surface to produce the desired thin film.
Despite improvements of CVD over previous deposition techniques, CVD has several disadvantages. For example, because CVD is flux-dependent, deposition conditions such as substrate temperature, pressure, and gas flow rate must be accurately and consistently maintained to produce a desired thin film of uniform thickness. Additionally, CVD tends to incorporate undesired reaction products into the deposited thin film, thereby diminishing the purity of the thin film.
Atomic Layer Deposition (ALD), which represents a variant of CVD, is a contemporary technology for depositing thin films now emerging as a potentially superior method of achieving highly uniform, conformal film deposition. ALD is a process wherein conventional CVD processes are divided into separate deposition steps to construct the thin film by sequentially depositing single atomic monolayers in each deposition step. The technique of ALD is based on the principle of the formation of a saturated monolayer of reactive precursor molecules by chemisorption. A typical ALD process consists of injecting a first precursor for a period of time until a saturated monolayer is formed on the substrate. Then, the first precursor is purged from the chamber using an inert gas. This is followed by injecting a second precursor into the chamber, also for a period of time, thus forming a layer on the wafer from the reaction of the second precursor with the first precursor. Then, the second precursor is purged from the chamber. This process of introducing the first precursor, purging the process chamber, introducing the second precursor, and purging the process chamber is repeated a number of times to achieve a layer of a desired thickness.
ALD thin films may be deposited using single wafer reactors with the reactive gas precursors injected into the process chamber horizontally. The horizontal gas precursor injection directs the gas precursors in a direction parallel to the surface of a stationary substrate. Arrangements in which the reactive gas precursors flow in a direction parallel to the substrate surface are desirable because they result in more uniform thin films than those deposited by gas precursors injected vertically in a direction perpendicular to the substrate surface. Nevertheless, a major disadvantage of a single wafer reactor is that it has a significantly diminished commercial value because of its relatively low throughput. Another disadvantage is that the process chamber must be purged before each individual gas precursor is introduced.
At least in part to overcome the commercial problems associated with single wafer reactors, multi-wafer reactors may be used for ALD processes in which the process chamber is partitioned into a plurality of process compartments. The substrates are rotated relative to the process compartments such that each substrate is sequentially transferred from one compartment to another. In each individual process compartment, each substrate is exposed to either a precursor gas or an inert gas. The rotating substrates are sequentially exposed to the different precursor gases and the inert gas. One characteristic disadvantage of these multi-wafer reactors is that multi-wafer reactors employ showerhead injectors that inject the precursor gases in an axially symmetric direction generally perpendicular to the substrate surface. As a result, although faster process times are achieved, multi-wafer ALD process chambers may produce thin films with reduced thickness uniformity.
Therefore, there is a need for a multi-wafer process chamber in which the reactive gas precursors are injected into each process compartment in a direction parallel to the top surface of the substrates.